(1) Field of the Invention
The present invention relates to an electron emitting device and an electromagnetic wave generating device, and particularly to an electron emitting device and an electromagnetic wave generating device which uses a carbon nanotube.
(2) Description of the Related Art
In recent years, as the need for improving performance of displays, electron microscopes, lighting systems, electromagnetic wave generating devices and the like intensify, development of electron emitting devices which use carbon nanotubes is underway for the purpose of realizing high-efficiency and high-power electron emitting devices which are used for the aforementioned equipment. The carbon nanotubes have properties that can achieve extremely high electric conductivity along the direction of each tube. Furthermore, since a carbon nanotube tip is sharper than tips of metal materials used as conventional electron emitting sources, the electric field intensity of the carbon nanotube tip is equal to or more than 10 times higher than the intensity of flat tips of metal materials. Thus, according to J-M Bonard et al., Solid-State Electronics Vol. 45 (2001), p. 893, by using carbon nanotubes in electron emitting devices, it is expected to be able to achieve high electron emission efficiency. In addition, since a carbon nanotube has high mechanical strength, using carbon nanotubes, it is also expected so that there is an advantage of realizing long-life and low-cost electron emitting devices.
In order to achieve high electron emission efficiency as expected, it is necessary to orient carbon nanotubes with respect to the direction where electrons are emitted and to emit, with certainty, the electrons from each carbon nanotube tip. The following two methods are reported as the carbon nanotube orientation growth technique: a method of generating a carbon nanotube by depositing, on a substrate, a metal layer which serves as a catalyst and causes gas-phase chemical reaction in hydrocarbon gas while keeping, at high temperatures, the substrate on which this metal layer has been deposited (for example, refer to Japanese Laid-Open Patent Applications No. 2001-15077, No. 2001-20071, and No. 2001-20072); and a method of generating an oriented carbon nanotube by removing Si when performing high-temperature annealing on SiC in a vacuum (for example, refer to Japanese Laid-Open Patent Applications No. 10-265208 and 2002-293522). Furthermore, FIG. 1 shows one of the examples of an electron emitting device structured using carbon nanotubes which are oriented in such a manner.
In the diagram, this electron emitting device has a structure including: an n− type SiC substrate 11; a carbon nanotube layer 12 which is formed with a high-temperature annealing method in a vacuum and is made up of oriented carbon nanotubes; an electrode 15; an ohmic electrode 17; and a voltage source 16 which applies a voltage between the electrode 15 and the ohmic electrode 17 which is on the SiC substrate 11. In such an electron emitting device, electrons emitted from the surface of the carbon nanotube layer 12 travels in an air-gap 14.
However, even in the electron emitting device in which the oriented carbon nanotubes are used as shown in FIG. 1, the work function which is a parameter that controls electron emission efficiency is extremely high, ranging from 4 to 5 eV. Thus, in order to achieve high electron emission efficiency, still high excitation energy is required. Consequently, this electron emitting device can not achieve high electron emission efficiency.
Here, as a technique which enhances electron emission efficiency, a technique is reported in W. S, Kim et al., Applied Physics Letters, Vol. 81 (2002), p. 1098, in which an electron beam is irradiated on carbon nanotubes of which orientation is imperfect but which are coated with MgO so as to emit secondary electrons. However, with this technique, because the nanotubes are oriented in random direction, in the case where the excitation energy of the electron beam is low, it is not possible to achieve the expected high electron emission efficiency. Therefore, even with this technique, it is not possible to achieve the high electron emission efficiency.
Thus, the object of the present invention is to provide an electron emitting device that can achieve high electron emission efficiency even in the case where excitation energy is low.